# std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::operator[]

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< cpp‎ | container‎ | flat map

T& operator[]( const Key& key ); | (1) | (since C++23)
T& operator[]( Key&& key ); | (2) | (since C++23)
template< class K > T& operator[]( K&& x ); | (3) | (since C++23)

Returns a reference to the value that is mapped to a key equivalent to key or x respectively, performing an insertion if such key does not already exist.

1) Inserts a `value_type` object constructed in-place if the key does not exist. Equivalent to return try_emplace(x).first->second;.
2) Inserts a `value_type` object constructed in-place if the key does not exist. Equivalent to return try_emplace(std::move(x)).first->second;
3) Inserts a `value_type` object constructed in-place if there is no key that transparently compares equivalent to the value x.
Equivalent to return this->try_emplace(std::forward<K>(x)).first->second;. This overload participates in overload resolution only if the qualified-id Compare::is_transparent is valid and denotes a type. It allows calling this function without constructing an instance of `Key`.

| Information on iterator invalidation is copied from here

### Parameters

key | - | the key of the element to find
x | - | a value of any type that can be transparently compared with a key

### Return value

1,2) A reference to the mapped value of the new element if no element with key key existed. Otherwise, a reference to the mapped value of the existing element whose key is equivalent to key.
3) A reference to the mapped value of the new element if no element with key that compares equivalent to the value x existed. Otherwise, a reference to the mapped value of the existing element whose key compares equivalent to x.

### Exceptions

If an exception is thrown by any operation, the insertion has no effect.

### Complexity

Logarithmic in the size of the container, plus the cost of insertion (if any) of an empty element.

### Notes

operator[] is non-const because it inserts the key if it doesn't exist. If this behavior is undesirable or if the container is const, `at` may be used.

`insert_or_assign` returns more information than operator[] and does not require default-constructibility of the mapped type.

### Example

Run this code

    #include <iostream>
    #include <string>
    #include <flat_map>

    void println(auto const comment, auto const& map)
    {
        std::cout << comment << '{';
        for (const auto& pair : map)
            std::cout << '{' << pair.first << ": " << pair.second << '}';
        std::cout << "}\n";
    }

    int main()
    {
        std::flat_map<char, int> letter_counts{{'a', 27}, {'b', 3}, {'c', 1}};

        println("letter_counts initially contains: ", letter_counts);

        letter_counts['b'] = 42; // updates an existing value
        letter_counts['x'] = 9;  // inserts a new value

        println("after modifications it contains: ", letter_counts);

        // count the number of occurrences of each word
        // (the first call to operator[] initialized the counter with zero)
        std::flat_map<std::string, int>  word_map;
        for (const auto& w : {"this", "sentence", "is", "not", "a", "sentence",
                              "this", "sentence", "is", "a", "hoax"})
            ++word_map[w];
        word_map["that"]; // just inserts the pair {"that", 0}

        for (const auto& [word, count] : word_map)
            std::cout << count << " occurrence(s) of word '" << word << "'\n";
    }

Output:

    letter_counts initially contains: {{a: 27}{b: 3}{c: 1}}
    after modifications it contains: {{a: 27}{b: 42}{c: 1}{x: 9}}
    2 occurrence(s) of word 'a'
    1 occurrence(s) of word 'hoax'
    2 occurrence(s) of word 'is'
    1 occurrence(s) of word 'not'
    3 occurrence(s) of word 'sentence'
    0 occurrence(s) of word 'that'
    2 occurrence(s) of word 'this'

### See also

at | access specified element with bounds checking (public member function)
insert_or_assign | inserts an element or assigns to the current element if the key already exists (public member function)
try_emplace | inserts in-place if the key does not exist, does nothing if the key exists (public member function)

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